With development of the Micro-Electro-Mechanical System (MEMS) technology, bi-material microcantilever beam structures are finding more and more applications, such as biosensors, arrays of micro-mirrors, capacitive infrared detectors, thermo-mechanical infrared detectors, and the like. These applications are based on a principle that a physical factor to be sensed is applied to the bi-material cantilever beam structure to cause deformation thereof. The deformation has a magnitude reflecting a strength of the factor to be sensed, and thus can be detected electrically or optically so as to read out the factor to be sensed.
However, practically manufacture processes tend to cause residual stress in the bi-material cantilever beam structure, the bi-material cantilever beam structure suffers an initial deformation after being released due to the mismatch of residual stress in two material films. Thus, a resultant device will have a decreased sensitivity or even fail. Therefore, in order for the device to function effectively, it is desirable to adjust the residual stress in the two material films to achieve stress matching.
There are mainly four types of techniques to adjust the film stress as reported. A first one is to adjust particular process parameter(s) in a film deposition apparatus for film stress adjustment, so as to achieve stress matching between the two material films. This method is applicable to the stress adjustment in various films, but it is complicated to operate and is time-consuming to develop because there are so many process parameters. A second one is to perform ion implantation and annealing in a deposited film for stress adjustment in the film. However, this method usually needs to perform the ion implantation with a high energy at a high dose to achieve stress matching, which may destroy a lattice structure of the film. Further, implanted ions may cause variations of original characteristics of the film. Such disadvantages limit applications of the method. Also, the incorporated additional processes caused an increased development cost and cycle. A third one is to introduce a stress gradient in a film by varying process parameters in depositing the film so as to achieve stress matching with a different film. This method is widely applicable. However, it is complicated to operate and is time-consuming to develop because there are so many process parameters. A fourth one is to release stress from a double-material beam by thermal cycling for stress adjustment. This method incorporates an additional process, resulting in an increased development cycle. Also, the thermal cycling generally means a high temperature, and thus limits applications of the method in some devices with temperature sensitive structures.
In view of the above, the existing techniques for matching the stress in the double-material film are not good enough to meet current trends of fables design and manufacture of MEMS devices.